Conventional coal-burning boiler plants and industrial furnaces combust coal in a reaction zone directly within the furnace. They are normally operated at an overall stoichiometry greater than one. This eliminates smoking but results in generation of substantial quantities of the oxides of nitrogen and the oxides of sulfur, as well as relatively high production of particulates, which require the use of high efficiency particulate collection devices such as baghouses. Such furnaces have relatively low energy release per unit volume, therefore requiring large volume "fire boxes" for burning of the fuel and extracting energy from the flame.
In recent years, oil prices have increased by about a factor of seven. Many electric-utility boiler plants and industrial furnaces were caught in a cost squeeze. Conversion of these boilers and furnaces with retrofit systems to burn coal rather than oil or gas, could provide very substantial energy-cost savings. But, attempting to burn coal in multi-megawatt boilers originally designed and constructed for oil or gas presents several difficulties that have been thought to be insurmountable: Slag and fly-ash from coal burning in such boilers would coat furnace and convective tubes, sharply reducing efficiency; uncontrolled emission of sulfur oxides (herein SO.sub.x) and/or nitrogen oxides (herein NO.sub.x) is prohibited by federal and local agencies in the urban and semi-urban locales where electricity-generating boiler plants are commonly located; in addition, most often the space available for installation of coal handling and combustion equipment is severely limited; and, boilers originally designed for oil and gas usually have no provision for ash collection and disposal.
We have developed a process and apparatus, suitable for retrofit installation on pre-existing boilers and furnaces, that removes most of the non-combustible mineral constituents of the fuel (e.g., coal) while combusting the fuel at heretofore unrealizable power densities and avoiding excessive generation of acid rain precursors, such as SO.sub.x and NO.sub.x. The desiderata, and performance characteristics of our invention, are:
High power density: about 1.0 million Btu/hour per cubic foot of volume of the primary combustion chamber.
Low NO.sub.x : Consistently less than 450 ppmv and, preferably, less than 250 ppmv in the gases emitted into the atmosphere.
Removable of Noncombustibles: Capture, and removal from the gaseous products of combustion, of 80% to 90% of the noncombustible-minerals content of the fuel before the gaseous products are conducted to the boiler or other heat-utilization equipment, depending on the requirements of the specific end-use equipment.
Carbon Carryover: Conversion of substantially all carbon to oxides of carbon before the gaseous products pass to the boiler or other heat utilization equipment.
Durability: Protection of the walls of the combustor so that deleterious corrosion and/or erosion of the walls is kept within commercially acceptable limits.
Thermal Efficiency: Delivery to the end-use equipment of a gaseous-products stream having about 85 to 90 percent of the chemical potential energy of the carbonaceous fuel. Preferably this energy is delivered partly as sensible heat and partly in the form of carbon monoxide and hydrogen contained in the gaseous products and readily combustible, to completion, in the end-use equipment.
Sulfur Capture: Removal of 80-90% of the sulfur-bearing constituents of the fuel from the gaseous products before the gaseous effluents pass from the heat-utilization facility into the atmosphere.
U.S. Pat. No. 4,217,132 to Burge et al., incorporated herein by reference, describes an apparatus for combusting carbonaceous fuel that contains noncombustible mineral constituents, separating such constituents as liquid slag and conveying a stream of hot combustion products to a thermal energy utilization equipment, such as a boiler. In the Burge et al. apparatus solid carbonaceous fuel (e.g., powdered coal) is injected into a combustion chamber and, simultaneously, a stream of oxidizer (e.g., preheated air) is introduced into the chamber to produce high velocity swirling flow conditions therein suitable for centrifugally driving most of the liquid slag to the inside walls of the chamber. Another system meeting, in part, the foregoing objectives is described in copending patent application Ser. No. 788,929 filed October 18, 1985 incorporated herein by reference (subsequently abandoned).
The apparatus described in patent application Ser. No. 788,929 relates to improvements in slagging combustors, resulting from extensive study and development including recognition of requirements peculiar to adapting slagging combustors to industrial furnaces and electric-utility boilers originally designed and constructed to use oil and/or natural gas.
Our invention is directed to further improvements in slagging combustion systems belonging to the same general class as that disclosed by Burge et al. and copending patent application Ser. No. 788,929 and, more particularly, to reduction of SO.sub.x and NO.sub.x emissions while simultaneously meeting the other desiderata described above.